Avalanche photodiodes (APDs) are important components in low-cost optical receivers. However, due to their long buildup time, current APDs typically are not able to meet the requirements for high bit-rate telecom systems. Currently, there appears to be no commercially available telecommunications APDs that can operate at the rate of 40 Gbps. A dynamic biasing method has been shown to be a novel solution that potentially improves the speed and sensitivity of APDs needed for 40 GB/S light-wave systems and beyond. Dynamic biasing of an APD is a non-constant bias of the APD.
A difficulty in implementing dynamic biasing APDs is their parasitic capacitance that causes a dynamic-bias signal to be injected into the optical current generated by the APD. See FIG. 1, where FIG. 1 is a schematic representation of a standard APD 105 with parasitic capacitance 110. At high frequencies the injected bias current, which is unwanted and hence noise, can be orders of magnitude larger than the optical current (signal) and overwhelms it completely. This issue can be referred to as the current-injection problem associated with the dynamic biasing of APDs.
There are few existing techniques that have been suggested to address the issue of current-injection problem in dynamic biasing APDs. These include the use of a notch filter to eliminate the injected bias noise as shown in FIG. 2. FIG. 2 is a schematic representation of an APD 205 arranged in a dynamic biasing arrangement with APD 205 coupled to a notch filter 215. Notch filter 215 can provide a voltage output in response to an optical signal incident on APD 205. However, since the frequency of the dynamic bias is the same as the frequency of the optical signal (bit rate), the notch filter will degrade the quality of the signal.
Another technique includes the use of a dummy APD and differential signaling technique to eliminate the dynamic bias that appears as a common-mode signal, as shown in FIG. 3. FIG. 3 is a schematic representation of an APD 305 arranged in a differential signaling technique with a dummy APD 320 using a transimpedance amplifier 315. A dynamic bias generator 330 can provide a drive signal to a radio frequency (RF) differential amplifier 325 that is coupled to both APD 305 and dummy APD 320. APD 305 can be coupled to dummy APD 320 at an input to transimpedance amplifier 315, where transimpedance amplifier 315 can provide a voltage output in response to an optical signal incident on APD 305. Although the technique of FIG. 3 is a preferred choice over the notch filtering approach of FIG. 2, a differential amplifier with a good common-mode-rejection-ratio (CMRR) will be required.
Another technique includes the use of a dummy APD with differential biasing technique to address the issue, as shown in FIG. 4. FIG. 4 is a schematic representation of an APD 405 arranged in a differential biasing technique with a dummy APD 420 using a RF differential amplifier 415. APD 405 and dummy APD 420 can be coupled to the same bias sources with APD 405 coupled to an input to RF differential amplifier 415 and dummy APD 420 coupled to another input to RF differential amplifier 415. In this arrangement, transimpedance amplifier 315 can provide a voltage output in response to an optical signal incident on APD 305. Transimpedance amplifier 315 may provide a CMMR of 80 dB. However, creating perfectly aligned differential biasing signals at high frequency is normally difficult to create. Moreover, there will still be a large injected bias current that will be dissipated through the parasitic capacitances of APDs.